In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. For example, specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) have been developed within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Another result of the forum's work is the High Speed Downlink Packet Access (HSDPA). In HSDPA multiple users provide data to a high speed channel (HSC) controller that functions as a high speed scheduler by multiplexing user information for transmission over the entire HS-DSCH bandwidth in time-multiplexed intervals (called transmission time intervals (TTI)). HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following: shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
Several new features have been added for the long term High Speed Packet Access (HSPA) evolution in order to meet the requirements set by the International Mobile Telecommunications Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency is to introduce support for four branch multiple input multiple output (MIMO), e.g., utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio (SNR) users and improves the coverage for low SNR users.
In a MIMO transmission system information-carrying symbol vectors s are multiplied by an NT×r precoder matrix WNT×r. The matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix H. Each of the r symbols in vector s corresponds to a layer and r is referred to as the transmission rank. The precoder WNT×r may be selected from a predetermined and finite set of countable precoders known to both the base station as well as the UE, a so-called codebook. This restricts the base station in the choice of precoder and is usually coupled with feedback reporting from the UE which recommends a precoder to the eNodeB.
Ideal linear precoding requires full channel state information (CSI) at the transmitter, which may be possible only for time division duplex (TDD)-based systems but not practical for frequency division duplex (FDD)-based systems. Codebook-based precoding allows the receiver to explicitly identify a precoding matrix/vector based on a codebook that should be used for transmission.
In 3GPP LTE standard, separate codebooks are defined for various combinations of the number of transmit antennas and the number of transmission layers. The latter is also called rank information or rank indicator (RI). For example, for four transmit antennas a total of sixty four precoding vectors and matrices are defined. Also for each rank in the codebook for the scenarios of RI=1, 2, 3, and 4, there are sixteen elements per rank are defined. The 3GPP standard does not specify what criteria the UE should use to compute the RI and/or the optimum precoding matrices/vectors. FIG. 1 shows the message sequence chart between a base station and UE.
The UE estimates the channel state information such as RI/PCI/CQI based on the pilot channel symbols. This information is sent to the NodeB via a feedback channel (e.g., HS-DPCCH). Once this information is received, a scheduler at the NodeB decides which modulation, coding scheme, PCI and RI is to be used for the data traffic channel. This information is sent to the UE via a downlink control channel, and data transmission starts thereafter.
The current HSDPA system (Release 7-1) supports 1 or 2 transmit antennas at the Node-B. For these systems, from channel sounding, a UE measures the channel and reports the channel state information in one sub frame. Typically this report consists of a channel quality indicator (CQI) which explicitly indicates Rank indicator (RI), and a precoding control indicator (PCI). The UE sends this report periodically for every subframe (TTI). Once the Node-B receives this report it grants the modulation and coding, number of codes, rank and the precoding channel indicator to each specific UE based on the scheduler metric.
Currently a 4Tx transmissions scheme for HSDPA is being discussed in 3GPP. One issue extensively discussed for 3GPP is design of a precoding codebook. It has been decided to use a codebook with 16 elements for each rank. In this regard, see, e.g. R1-121761, Precoding Codebook Design for Four Branch MIMO System, Mar. 26-30, 2012, which is incorporated herein by reference in its entirety.
In general the NodeB has no control in selection of rank/precoding index/CQI reported by the UE. Moreover, in some cases, the UE feedback information is not useful for the NodeB. As an example, the NodeB may not have the appropriate power and/or codes for which to schedule the user in accordance with the UE rank information. In other instances, the NodeB has the capability to schedule the users with certain precoder elements. But in these cases the NodeB must inform the UE, regarding what rank information the NodeB prefers, what precoder elements the NodeB does not prefer, etc.
One method in coordinating selection of transmission parameters involves introduction of a codebook subset restriction in which the network (e.g., RNC) sends the bitmap to the UE through a higher order signaling (RRC) during the cell set up. The UE uses this bitmap when reporting the channel state information. An implementation of such method is described in detail in U.S. Provisional Patent application No. 61/683,665 “IMPLEMENTING CODEBOOK SUBSET RESTRICTIONS IN HIGH SPEED DOWNLINK PACKET ACCESS SYSTEMS, which is incorporated herein by reference in its entirety. More specifically, in an implementation of codebook subset restriction a UE is restricted to report PCI and RI within a precoder codebook subset specified by a bitmap parameter codebookSubsetRestriction configured by higher layer signaling. This information is sent through RRC signaling during the cell setup. The bitmap can specify all possible precoder codebook subsets from which the UE can assume the NodeB may be using when the UE is configured. The bitmap forms the bit sequence a63, . . . , a3, a2, a1, a0 where a0 is the LSB and a63 is the MSB and where a bit value of zero indicates that the PCI reporting is not allowed to correspond to precoder(s) associated with the bit. FIG. 2 shows an example message sequence which sets up this implementation of codebook subset restriction operation.
There are problems, however, with such an implementation of a codebook subset restriction solution. For example, once the bitmap sent to the UE, there is no mechanism whereby the RNC can change the bitmap. Thus, the full benefits of the codebook subset restriction functionality are not obtained.